Soft drinks are some of the most widely consumed beverages. Flavor oils, such as orange, lemon and peppermint oils, are often used in drinkable beverages as flavoring agents, since they contain volatile constituents with characteristic aroma profiles. Soft drinks may also contain a variety of hydrophobic components, such as clouding agents, weighting agents, nutraceuticals, oil-soluble vitamins, and oil-soluble antimicrobials. Due to the non-polar character of flavor oils and other hydrophobic ingredients, such ingredients are not typically dispersed directly into an aqueous phase because they would rapidly coalesce and separate through gravitational forces leading to a layer of oil on top of the product. Instead, these ingredients are usually converted into a colloidal dispersion consisting of flavor molecules encapsulated within small particles suspended within an aqueous medium, i.e., an emulsion.
The emulsions used in the beverage industry are typically divided into two groups: flavor emulsions and cloud emulsions. Flavor emulsions contain lipophilic compounds that are primarily present to provide taste and aroma to a beverage product (such as lemon, lime, or orange oils). On the other hand, cloud emulsions are used to provide specific optical properties to certain beverage products, i.e., to increase their turbidity (“cloudiness”). Cloud emulsions are typically prepared using an oil phase that is highly water-insoluble and that is not prone to chemical degradation, such as flavorless vegetable oils. In addition, the size of the droplets within cloud emulsions is typically designed so that the droplets are dimensioned such that strong light scattering occurs, but not too large to undergo gravitational separation. Cloud emulsions are often added to beverages that only contain a relatively low percentage of juice and provide a desirable cloudy appearance that hides sedimentation and ringing.
Generally, an emulsion consists of at least two immiscible liquids (usually oil and water), with one of the liquids being dispersed as small spherical droplets in the other. Emulsions are classified according to the relative spatial organization of the oil and water phases. A system that contains oil droplets dispersed within water is called an oil-in-water (O/W) emulsion, whereas a system that contains water droplets dispersed in oil is called a water-in-oil (W/O) emulsion. Currently, almost all of the emulsions used in the beverage industry are of the O/W type, although there may be advantages to using other emulsion types for some applications.
Beverage emulsions experience a range of environmental stresses during their manufacture, transport, storage, and utilization that may reduce their shelf lives. Examples include mechanical forces (e.g., stirring, flow through a pipe, centrifugation, vibrations, and pouring); temperature variations (e.g., freezing, chilling, warming, pasteurization, and sterilization); exposure to light (e.g., natural or artificial visible or ultraviolet waves); exposure to oxygen; variations in solution properties (e.g., pH and mineral composition of water). Exposure to these environmental stresses may promote emulsion instability through a variety of physicochemical mechanisms including loss of ingredient functionality (e.g., changes in solubility, surface activity, or stabilization capacity); acceleration of chemical degradation reactions (e.g., oxidation, polymerization, or hydrolysis); and acceleration of physical instability mechanisms, (e.g., flocculation, coalescence or Ostwald ripening).
Generally, emulsions are thermodynamically unstable systems that tend to break down over time due to a variety of physicochemical mechanisms, including gravitational separation, flocculation, coalescence and Ostwald ripening. Such instability mechanisms lead to a change in the structural organization of the various components within the system and can lead to detrimental changes in the physical stability of the beverage emulsion. Gravitational separation is one of the most common forms of physical instability in commercial beverage emulsions and may be in the form of creaming or sedimentation, depending on the relative densities of the oil droplets and the surrounding aqueous phase.
One such problem in beverage emulsions is creaming, which is the upward movement of droplets when they have a lower density than the aqueous phase, while sedimentation is the downwards movement of droplets when they have a higher density than the aqueous phase. Creaming is more prevalent since the oil phases used in beverage emulsions consist primarily of triacylglycerol and/or flavor oils, which have lower densities than water. A beverage emulsion may be prone to sedimentation if it contains very small oil droplets covered by relatively thick and dense interfacial layers.
Another problem that may occur in beverage emulsions is “ringing,” which is the accumulation of a visible ring of oil droplets on the top of a product, which may be visually displeasing to some consumers. The “ringing” effect is due to droplet creaming, which may have occurred because a population of droplets in the initial emulsion was too large, or because some droplet growth occurred during storage, e.g., due to flocculation, coalescence, or Ostwald ripening. Ostwald ripening is the process where the size of the oil droplets in an oil-in-water emulsion increases over time due to diffusion of oil molecules from small to large droplets through the intervening aqueous phase. The susceptibility of a beverage emulsion to Ostwald ripening is mainly determined by the solubility of the oil phase in the aqueous phase. Specifically, the higher the solubility of the oil phase in the aqueous phase, the more unstable the emulsion.
Generally, oil density determines the rate of particle creaming or sedimentation within emulsions. Specifically, the greater the density contrast between the droplets and surrounding fluid, the faster the rate of gravitational separation. The stability of a beverage emulsion to gravitational separation can therefore be improved by ensuring that the density of the oil droplets is similar to that of the surrounding aqueous phase. The densities of flavor oils and vegetable oils are typically considerably lower than those of water and aqueous sugar solutions and for that reason, droplets containing flavor oils and vegetable oils tend to move upward during storage leading to creaming, which is undesirable. The creaming rate in an emulsion may be reduced by decreasing the density contrast between the oil droplets and the surrounding aqueous phase.
Weighting agents are additives incorporated into the oil phase of certain types of beverage emulsions to inhibit gravitational separation of the oil droplets. The creaming and ringing issues in beverage emulsions may be reduced by decreasing the density contrast between the oil droplets and the surrounding aqueous phase. Weighting agents, which are typically hydrophobic components that have a density considerably greater than water, may be used to increase the density of the oil phase so that it matches that of the aqueous phase. Notably, beverage emulsions are one of the few products where these weighting agents can be used to reduce the creaming rate, because the oil phase content is typically very low (<0.1%).
A number of different weighting agents are known for utilization within commercial beverage products. Such weighting agents include brominated vegetable oil (BVO), sucrose acetate isobutyrate (SAIB), ester gum, and dammar gum. There are a number of different factors that will determine which of these different components is suitable for application within a specific beverage product. There are also limits on the amount of each type of weighting agent that may be legally used in beverages. There are also differences in the “labelfriendliness” of different weighting agents as some weighting agents may be perceived by the consumers as “more natural” than others.
BVO and ester gum are the most commonly used weighting agents in beverage emulsions. However, the amount of weighting agents that can be found in the final product is often regulated. For example, the use of BVO and ester gums is often limited to 15 and 100 ppm per serving, respectively. Such a low concentration of allowable weighting agents restricts the type of products that BVO and ester gum may be used in.
Brominated vegetable oil (BVO) is made when bromine is added to the double bonds of the triacylglycerol molecules in corn, soybean, cotton seed, or olive oil. In some cases, BVO is permitted for use at a level not exceeding 15 ppm per serving. However, there has recently been consumer concern about the presence of BVO in soft drinks and even though BVO is still permitted for use in the United States, it is undesired by some consumers.
Ester gum is a hydrophobic polymer made when glycerol is esterified to gum rosin. It is normally supplied as a crystalline solid that can be incorporated into the oil phase. Ester gum may be considered by some to be an artificial food ingredient due to the esterification step used in its preparation, but it is derived from natural components (gum rosin) and non-animal glycerol, and in some cases is acceptable for use at a level not exceeding 100 ppm per serving. Ester gum performs similarly to BVO, but a greater concentration of ester gum has to be added to the oil phase in order to raise the density. Dammar gum is a natural weighting agent that is isolated from an exudate of Caesalpinaceae and Dipterocarpaceae shrubs. Dammar gum is approved for use in some countries, but does not have the generally regarded as safe (GRAS) status in the United States.
Sucrose acetate isobutyrate (SAIB) is a synthetic weighting agent produced by the esterification of sucrose with acetic and isobutyric anhydrides. SAIB is usually supplied in the form of a high viscosity transparent liquid that can be mixed with the oil phase prior to homogenization. The use of SAIB in beverages is currently permitted in amounts up to 300 ppm per serving in some cases.
There are several drawbacks with conventional weighting agents such as SAIB, BVO, and GEWR. First, use of SAIB, BVO, and GEWR in beverages can be limited to 300, 15, and 100 ppm per serving, respectively, in some instances. Also, SAIB, BVO, and GEWR are ingredients that may be undesired by some consumers. In addition, the densities of SAIB, BVO, and GEWR are 1.146 g/ml, 1.24-1.33 g/ml, and 1.08 g/ml, respectively, which may be too low to properly weight an oil droplet. For example, sugar-sweetened ready-to-drink or liquid concentrated beverages can have aqueous-phase densities ranging from about 1.05 to about 1.25 g/ml and a flavor/cloud oil-phase density of about 0.9 g/ml. Due to the above-mentioned limits on use and densities of SAIB, BVO, and GEWR, these weighting agents may not weight the oil phase to the aqueous phase's density, thereby causing the beverage to undesirably form a cream layer over its shelf-life.